It has long been recognized that in many voice communication networks, the far end has an annoying tendency to return to the near-end speaker a delayed replica of his voice transmissions. Such far-end echo is especially bothersome when it occurs at a delay of about 40 ms or more, because at such delays, the echo tends to be distinctly perceived by the near-end speaker as distracting noise. Thus, far-end echo poses especially severe problems for those types of network whose operation entails such relatively large delays. These include satellite networks, and at least some networks that perform coding and compression of speech.
Devices are, in fact, available that would enable the far-end speaker to suppress or cancel the near-speech component that he is unintentionally returning to the near end. However, there will be cases when the far-end speaker is not using such a device. Moreover, even if such an echo-suppressing or echo-canceling device is being used at the far end, it may not be completely effective for removing echo. Thus, in many cases there will be at least residual echo returned to the near end.
As a consequence, it will often be desirable for the near-end speaker to operate a device that can reduce those components of near speech that are returned to the near-end speaker after traversing a round trip through the remote communication network.
An early nonlinear processor for reducing echo was described in O. M. Mracek Mitchell and D. A. Berkley, "A Full-Duplex Echo Suppressor Using Center-Clipping," Bell System Technical Journal 50 (1971), pages 1619-1630. When this article was published, echo cancellers were not yet in use. In the article, the authors described a sub-band center clipper for use as a stand-alone device to replace a conventional (at the time of publication) echo suppressor at the far (i.e., receiving) end. This center clipper had no adaptations for situations where there is a substantial echo delay.
U.S. Pat. No. 5,274,705, issued to Younce et al., describes a more recent effort to suppress residual echo using a device at the far (receiving) end. Echo that has not been completely removed by a conventional echo canceler is further removed by a non-linear processor. In this non-linear processor, an estimate of the background noise level is used to set a fullband, noise-transparency threshold. Transmissions falling below this threshold are transmitted in order to mask residual echo and to avoid unnatural-sounding interruptions of the background noise. This technique also uses the energy in an echo replica, based on an estimated gain for the echo path, to set a time-varying threshold for fullband center clipping.
The Younce technique may, in some cases, fail to achieve a satisfactory degree of echo control. For example, residual echo that survives the center-clipping process will extend over the full frequency band, and thus may be recognizable as speech (and hence, be distracting) even at very low signal-to-noise ratios. Moreover, full-band noise transparency is disadvantageous because narrow-band noise, such as power-line hum, will tend to raise the noise-transparency threshold across the full frequency band. This can result in the unintended transmission of echoes which are masked by noise only in a limited frequency range.
Practitioners in this field have recognized that a device situated at the near (transmitting) end can be used to reduce far-end echo, if it compensates for the delay incurred by transmission of the echo over a round trip through the local and remote networks. For example, International Patent Application PCT/AU93/00626 (International Publication WO94/14248), by J. Portelli, describes the use of a conventional echo canceller at the near (transmitting) end. Because there may be a substantial delay between the transmission of the near speech and the arrival of the echo that is to be cancelled, this echo canceller is operated in conjunction with a delay device which is programmed, prior to installation, to provide a fixed, compensatory delay. In the echo canceller, a fullband adaptive transversal filter generates a subtractive replica of the echo. However, certain factors may prevent this system from providing an entirely satisfactory remedy. For example, the accuracy of the echo replica is limited by line noise. This may reduce the effectiveness of the echo canceller. Moreover, circuit multiplication or compression equipment between the local and remote networks can distort portions of the echo signal, leading to incomplete suppression. This system may also suffer degraded performance due to phase roll (e.g., from analog transmission facilities), or due to quantization noise and nonlinearities introduced by speech coders in digital transmission systems.
Thus, practitioners in the field of echo control have hitherto failed to provide a fully satisfactory method that can be employed in the local network to reduce residual far-end echoes.